BEIRUT — Syrian rebels knocked down army defenses and moved in on the country’s second-largest airport Wednesday, reportedly killing more than 40 soldiers and bringing them closer to what could be their biggest conquest since the beginning of the civil war.

Control of Aleppo International Airport and a military air base next to it would be a huge strategic shift for Syria’s northeastern region, giving the opposition a potential air hub enabling aid and other flights.

Still, activists said it could be days before the rebels would be able to push their way into the airport, 4 miles from the contested city center, and even then, it was unclear whether they would be able to retain control of the sprawling facility for long.

The country’s air space is firmly controlled by the government, which uses its warplanes indiscriminately to bomb rebel strongholds.

The advance on the airport, which stopped handling any flights weeks ago because of the fighting, comes on the heels of other strategic gains. Rebels this week captured the nation’s largest dam and a military base near Aleppo. They have also brought the fight closer to Damascus, seat of President Bashar Assad’s regime, moving to within a few miles from the heart of the city.

‘‘There has been some extremely significant advances by the rebels in the past few days. There is real fear and flagging morale among regime forces in the region,’’ said Muhieddine Lathkani, a London-based member of the Syrian National Council opposition group.

The government tried to reverse the gains with a series of airstrikes in several locations across the country Wednesday. In Jobar, a rebel stronghold in northeastern Damascus, 13 people were killed in government shelling, according to the Observatory. Fighter jets also carried out airstrikes on rebel positions in the central province of Homs, it said.

The rebels have been pushing their way into the capital since last week. The foray marks the opposition’s second significant attempt to storm Damascus since July, when the rebels captured several neighborhoods before being swept out by a swift government counteroffensive.

Since then, the regime has buckled down in Damascus, setting up checkpoints and controlling movement in and out of the city. The heavily defended city so far has been spared the kind of violence that has devastated whole neighborhoods in other major cities such as Aleppo and the central city of Homs.

The rebels have had the most success in northern Syria, and have captured large parts of Aleppo, Idlib, and Raqqa provinces since the Syrian conflict began in March 2011.

Rebels have been attacking the civilian airport in the city of Aleppo for weeks, and on Tuesday overran large parts of the ‘‘Brigade 80’’ base that protects the facility.

By Wednesday, the Observatory said the rebels were ‘‘almost fully in control’’ of the base. Rami Abdul-Rahman, the group’s director, said more than 40 government troops were killed in the fighting, including two brigadier generals, a colonel, and two lieutenant colonels. He also said an unknown number of rebels died. The report could not be independently confirmed.

The base is about a mile from the civilian airport and the adjacent air base known as Nairab. Both have their own defenses in addition to the protection provided by Brigade 80.

Lathkani said it would be one of the opposition’s most significant gains of the civil war if the rebels capture Aleppo airport, which according to its website is capable of handling 2.5 million passengers a year.

He said rebels were aiming to declare northern Syria a ‘‘liberated area’’ from which they would one day announce a transitional government.

Also Wednesday, Syria’s former Foreign Ministry spokesman made his first comments since disappearing in December, saying he left the country because ‘‘of the polarization and violence that left no place for moderation and diplomacy.’’

Jihad Makdissi said in a statement that the uprising has ‘‘legitimate demands.’’

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